This invention is directed to an internal combustion engine control unit and, more particularly, to a controller for an engine operable with both stratified and homogeneous combustion, switched according to engine operating conditions.
A control unit has been proposed (for example, in Japanese Patent Kokai No. 62-110536) for electronic control of a throttle valve (referred to hereinafter as an electronic controlled throttle valve) used in an internal combustion engine. The control unit sets a target engine torque based on accelerator operation degree and engine speed and controls the degree of opening the throttle valve to obtain the target engine torque.
However, such a conventional unit is arranged to determine the degree of opening of the throttle valve based on target engine torque and engine speed only. Thus, such a conventional unit cannot be directly applied to an internal combustion engine having its air-fuel ratio controlled according to engine operating conditions.
It is required to change both the degree of opening of the throttle valve and the amount of fuel metered to the engine in order to change the air-fuel ratio, while maintaining engine speed and torque. For example, the required amount of air permitted to enter the engine is greater and the required amount of fuel metered to the engine is smaller at lean air-fuel ratios as compared to the stoichiometric air-fuel ratio. Japanese Patent Application No. 8-36902 (filed Feb. 23, 1996) proposes control of the engine torque to its target value while maintaining the air-fuel ratio at its target value. The entire contents of this Japanese Patent Application is incorporated herein by reference.
In this design, if the target equivalence ratio (which is proportional to the reciprocal of the air-fuel ratio) changes, the required amount of air to the engine and the required amount of fuel to the engine will change accordingly. The amount of air to the engine changes with a delay because the amount of air to the engine is delayed due to the volume of the intake system, even though the throttle valve is controlled to its target position according to the changed target equivalence ratio. If the amount of fuel to the engine is controlled according to the amount of air to the engine based on the changed target equivalence ratio, the amount of fuel to the engine will change to a great extent, which in turn changes the torque in a stepped fashion at an initial stage of the changing operation.
For this reason, in conventional control, phase delay is corrected in such a manner as to change the target equivalence ratio with a delay corresponding to the delay with which the amount of air to the engine changes. The above application discloses a phase delay correction for the amount of fuel calculated based on the actual amount of air to the engine as well as a direct phase delay correction for the target equivalence ratio. This technique is similar to a target equivalence ratio phase delay correction. This technique can change the equivalence ratio gradually toward its target value while smoothly changing the torque at a constant rate.
A new technique has recently been developed for use with a spark ignition type gasoline engine to permit combustion at very lean air-fuel ratios so as to improve fuel economy and exhaust gas purifying performance to a great extent by injecting fuel directly into the combustion chamber. Such a technique involves injecting fuel during the compression stroke at low and intermediate load conditions to form a combustible mixture in a stratified condition around the spark plug for operation with stratified combustion. Detailed description of such techniques is set forth in Japanese Patent Application P9-135269 (filed May 26, 1997), P9-137369 (filed May 28, 1997), P9-132673 (filed May 23, 1997), P9-129053 (filed May 20, 1997), and corresponding U.S. patent application Ser. No. 09/081,071 , entitled "Direct Injection Gasoline Engine with Stratified Charge Combustion and Homogeneous Charge Combustion", filed under Attorney Docket Number 040679/0625 on May 19, 1998. The entire contents of these five documents are incorporated herein by reference.
In order to ensure the required torque with a limited cylinder volume, the engine is also required to operate with homogeneous combustion by injecting fuel during the intake stroke to form a homogeneous mixture for operation with homogeneous combustion. Fuel can be injected directly into the cylinder or into another fuel injector in the intake port(s).
It is, therefore, a known practice to make a switch between stratified and homogeneous combustion according to the engine operating conditions.
One may consider making a delay correction for the target equivalence ratio according to the delay with which the amount of air to the engine changes, in order to change the torque smoothly while the target equivalence ratio changes when a change is made between stratified and homogeneous combustion. However, operation with stratified combustion and homogeneous combustion are different from each other with respect to optimum external EGR (exhaust gas recirculation) ratio (representing exhaust gas which is recirculated by a separate EGR system) and internal EGR ratio (representing exhaust gas left in a cylinder) as well as with respect to target equivalence ratio. If these factors are unbalanced, emissions and driving stability can be degraded.
In greater detail, it is required, during operation with stratified combustion, to perform external EGR for NOx reduction. EGR lowers the combustion temperature which in turn lowers NOx emission. Since the air-fuel ratio is extremely lean during the operation with stratified combustion and during operation with homogeneous combustion just after a change to operation with homogeneous combustion, external EGR is inhibited during a change from stratified to homogeneous combustion to avoid deterioration of combustion performance and power output. It is possible to reduce the emission of NOx while improving fuel economy by leaning the air-fuel ratio to a sufficient extent since the engine is operating under conditions where NOx emission level is relatively low.
Upon a change from operation with stratified combustion (very lean) to operation with homogeneous combustion (lean), however, the residual EGR gas in the intake system may degrade the combustion performance and thus the acceleration performance even though a command is outputted to stop EGR at the same time as operation with stratified combustion is completed. An attempt to attach importance to the acceleration performance by changing the equivalence ratio promptly will cause the equivalence ratio to change at a greater rate than the EGR rate. This also degrades combustion performance. Even when external EGR is stopped early enough during operation with stratified combustion, the influence of the interior EGR gas residing in the combustion chamber during the operation with stratified combustion may remain after the change to homogeneous combustion to degrade combustion performance.
Since the amount of air to the engine increases with a delay, although the target EGR ratio increases with a delay upon a change from operation with homogeneous combustion to operation with stratified combustion, it is possible, with no problem, to perform a sufficient degree of EGR in a transient condition just after a change to operation with stratified combustion.